The fuel assemblies of water-cooled nuclear reactors, such as pressurized-water nuclear reactors, comprise a framework in which fuel rods of great length are disposed in order to form a bundle. The framework comprises spacer grids which are spaced relative to one another along the length of the assembly and connected together by guide tubes. End joining pieces are connected to the end of the guide tubes, which are longer than the fuel rods.
Each of the spacer grids comprises an assembly of cells each intended to receive a fuel pencil and disposed in a regular network, generally a squared mesh.
At the level of each of the cells intended to receive a fuel rod, the spacer grids comprise means for gripping the rod, while also ensuring both transverse retention and longitudinal retention of such rod. These gripping means generally consist of bosses projecting inwards relative to the walls of the cell of the grid and of springs consisting of resilient elements formed in the metal of certain walls of the cells of the grid or, alternatively, connected to these walls.
After a greater or lesser residence time in the tank of a light-water nuclear reactor, the rods of the fuel assemblies may become brittle due to oxidation phenomena or due to transformations suffered by their sheaths under irradiation. The sheaths of certain rods may be perforated or torn by a peripheral guide fin of a spacer grid which has accidentally been folded inwards or by a foreign body conveyed by the fluid for cooling the reactor circulating in contact with the fuel rods. The fins or the foreign bodies may be encrusted in the sheath which is then deformed or perforated in the corresponding zone.
It is necessary to perform repairs on fuel assemblies comprising rods whose sheath is perforated or torn, by removing these rods and replacing them with new ones. Removal is performed by pulling the rod via its upper end plug after having removed the upper joining piece of the assembly. Rods may also be removed and replaced by removing the lower joining piece.
To this end, fuel assemblies of recent design comprise joining pieces whose elements for fixing on guide tubes may be removed without difficulty.
All the operations for removing and replacing rods in the spent assemblies of a nuclear reactor are performed under a certain depth of water which is greater than three meters, inside a storage pool in which the fuel assemblies are placed in a vertical position. The operators carrying out the repair operations from the edge of the pool are thus protected against radiation from the spent assemblies.
The fuel rods whose sheath is perforated or torn are liable to break during their removal, the gripping elements of the spacer grids exerting a certain axial retention force on the rods which it is necessary to overcome in order to perform the removal.
There then remain pieces of rod inside the fuel assembly which can no longer be removed by the usual means.
The peripheral rods of the assembly, i.e., the rods placed in the four outer rows which are adjacent to the frame of the spacer grids, are the most exposed to impacts with foreign bodies and to the effects of penetration by the guide fins of the spacer grids. These rods are thus the most exposed to breakage during the removal operations.
To date, there was no known device or method making it possible to remove the peripheral rods of a fuel assembly comprising several successive pieces in the direction of their length, the assembly being placed under water in a storage pool.
The spent and damaged assembly must be replaced by a new assembly and dismantled or repaired by using complicated equipment which is available only in nuclear fuel reprocessing plants.
This results in increased operating costs for the nuclear reactor.